It shouldn't really make much difference for most blades, but if you had a really, really sharp blade, it would be sharper at lower temperatures. Imagine a blade that is only one atom thick at it's very tip (they do exist!). If the temperature is very low, this can be stable and well localized. As you increase the temperature, the atoms are going to start vibrating more and more. Their "positions" will smear out a little bit, so even if on average the atoms are all in the right place, the edge will appear thicker because the atoms are moving back and forth. Once the temperature gets hot enough, the atoms at the very edge will have enough energy to leave (either going into gas phase, or moving to where the blade is thicker, where it gets to bind to more atoms than it could at the very edge)

Discounting blades that are one atom thick at the edge; if you have a very sharp knife with a cold blade, heat it so it becomes less sharp, then sharpen it; when it returns to its original temperature will it be sharper than it was originally?

An atom at the edge is held in place by chemical bonds with one or more atoms "further in" the blade. Dave, your intuition is correct that the arrangement is only meta-stable. Given enough energy, the atom will migrate to a more stable position.